Flat Panel Displays (FPDs) have a wide range of consumer, industrial and military applications and are projected to exceed $20 billion dollars by the year 2000. Of the many types of FPD technologies, Liquid Crystal Display (LCD) technology leads the pack and is most mature. The profit margins in the LCD market are becoming increasingly low, because of the changing market. There is an increased push to reduce the material cost in making FPDs which is estimated to be about 40 to 50% of the total display cost. There is a thrust towards looking at new materials, decreasing weight, reducing display thickness, improving viewing angles and lowering back illumination.
One of the key challenges to making the next generation, low weight FPDs is replacement of glass displays with plastic displays. Plastic displays are lower weight, shatter resistant and can provide equivalent transmission properties to that of glass. However, plastics such as polycarbonates and polyethylene terephthalates (PET) are not scratch and abrasion resistant. It is important to provide a hard coating to the plastic with good transmission and refractive index matching to effectively function as flat panel displays. Another disadvantage of plastics is they are permeable to liquids and vapors, which is undesirable. Permeation of water vapor, oxygen, liquid crystals and other gases could ruin the electronics behind the flat panel display, if proper barrier protection is not provided. Thus the hard coating also needs to be impermeable to liquids, vapors and solvents used in the FPD manufacturing.
The markets for plastic substrate FPDs is quite huge, prominent being the LCD market. Plastic substrate displays could be used in LCD applications, which currently use soda-lime glass displays. This is a sizable portion of the display market and can be as high as 5 billion square inches by year 2000. The LCD applications, where hardcoated plastic displays could be used, include consumer applications such as clocks, watches, calculators, games, personal digital assistants (PDAs), phones, televisions, camcorders, cameras and industrial applications include PCs, medical inuments, facsimiles, thermostat meters, industrial displays, test equipment etc.
There are a number of hardcoats that are available in the industry. The predominant coatings which might be considered as hardcoats are DLC and polysiloxane dip coatings. Conventional DLC coatings consist of carbon and hydrogen with no silicon and oxygen. Polysiloxane coatings are deposited by spin coating, spraying or dipping followed by a curing step or UV hardening.
Dip coatings, are wet chemical processes which require significant handling and several hours to go through the coating process. Also, the chemicals used are partially toxic and combustible, making them increasingly difficult to handle due to environmental considerations. The present invention has the potential of depositing coatings at high deposition rates, which is cost-effective, requires minimum handling and is benign to the environment.
The present invention is a low temperature process capable of depositing on heat sensitive plastics such as polycarbonates and polyethylene terephthalates. The substrate temperature does not exceed 70.degree. C. during normal operation. However, active cooling of the substrate temperature is possible and the substrate temperature can be maintained at room temperature. Because the present invention involves a low temperature process, there is no thermal mismatch issue between the substrate and the hard coating. DLC processes typically require higher temperatures (as high as 200.degree. C.) and hence adapting the DLC technology to coating low temperature materials is a significant problem. The present invention can be deposited on substrate materials as thin as 0.075 mm while some dip coating polysiloxane processes require rigid substrates (minimum thickness.about.1 mm).
DLC coatings exhibit high compressive stresses (sometimes 5 times that of the present invention) resulting in poor adhesion to substrates. The poor adhesion of DLC necessitates the use of interlayers to improve adhesion, which is not required in the present invention.
The present invention yields an extremely hard coating compared to polysiloxane dip coatings. The polysiloxane hard coatings are not as abrasion resistant as glass. DLC coatings are harder than the present invention, but because of their increased hardness they exhibit high internal stress and hence require interlayers to promote adhesion.
Petrmichl et al (U.S Pat. No. 5,618,619) discusses an abrasion resistant coating deposited using an ion-assisted process with nanoindentation hardness of 2 to about 5 GPa. The coatings of Petrmichl et al are softer than the present invention and their effectiveness as a barrier coating is unknown. Also the invention of Petrmichl et al teaches the use of only siloxane or silazane with oxygen. This limits the highest coating hardness that can be achieved. The use of hydrogen in addition to oxygen and siloxane or silazane precursors is novel in the present invention and allows achieving higher hardnesses. Petrmichl et al does not indicate the nature of bonding between the C, H, Si and O in the material. It is well known to one skilled in the art of materials science and engineering that the bonding between these coatings influences the properties of the material.
Lin et al.sup.1. have reported on depositing SiO.sub.2 -like films using siloxane and oxygen as precursor materials by high-density microwave electron cyclotron resonance discharge. The resulting coating material is different from the present invention and consists mainly of a silicon dioxide. Fourier Transform Infrared Spectroscopy (FTIR) indicates presence of a SiO.sub.2 -like band (towards higher wavenumbers and sharper Si--O stretching) and small amount of Si--CH.sub.3 bonding. The high-density plasma approach utilized by Lin et al is probably responsible for the different type of bonding, different material density (presence of micropores) and higher concentration of oxygen observed in the films. Even though Lin et al have not reported the barrier properties, a microporous coating is expected to have poor barrier properties. The hardness range of this material is unknown.
Dorfman et al (U.S Pat. Nos. 5,466,431 and 5,352,493) describe a diamond-like coating consisting of C, H, Si and O. The optical transparency of the coatings of Dorfinan et al. in the UV-visible range is limited and is highly dependent on coating thickness. Thus, the coatings of Dorfman et al do not have the same degree of transparency as the coatings of the present invention.
Thus, there is a need in the industry for a coating for plastic substrates, and more particularly for flat panel displays which can be deposited on the plastic substrate by a low temperature process at a high deposition rate; and which is cost effective and benign to the environment; and which produces a coated product that is hard, optically transparent, impermeable, scratch and abrasion resistant and that have good adhesion.